1. Field of the Invention
This invention relates to communication systems. More particularly, the invention relates to IBOC systems using OFDM with timing and frequency offset correction.
2. Description of Prior Art
IBOC systems enable digital signals to fit within the bandwidth or frequency allocation of existing AM and FM stations. IBOC systems preserve frequency spectrum by using existing allocations efficiently to deliver digital sound without disrupting AM and FM broadcasting. IBOC systems are subject to multipaths for the interference of signals, delayed upon themselves, due to reflections from man-made structures, such as buildings or from natural features, such as hills and trees. One method of minimizing the effect of multipaths is the use of Orthogonal Frequency Division Multiplexing (OFDM) technique. OFDM is a form of multi-carrier modulation where carrier spacing is carefully selected so that each sub-carrier is orthogonal to the other sub-carriers. In OFDM, the total signal frequency band is divided into N non-overlapping frequency sub-channels. Each sub-channel is modulated with a separate symbol and, then, the N sub-channels are frequency-division multiplexed. There are three schemes used to separate the modulated sub-carriers: (1) The use of filters to completely separate sub-bands; (2) the use of staggered quadrature amplitude modulation (QAM) to increase the efficiency of band usage; and (3) the use of discrete Fourier Transforms to modulate and demodulate parallel data. The orthogonality of the sub-channels can be maintained and individual sub-channels can be completely separated by Fast Fourier Transforms (FFT) at the receiver when there are no Intersymbol Interference (ISI) and Intercarrier Interference (ICI) introduced by transmission channel distortion. One way to prevent ISI is to create cyclically extended guard intervals where each OFDM symbol is preceded by a periodic extension of the signal itself. When the guard interval is longer than the channel impulse response, or the multipath delay, the ISI can be eliminated. Using cyclic prefix at the transmitter, one can determine OFDM frame boundaries. If there is a difference in the clock frequencies at the transmitter and receiver, there are two problems. First, the difference causes an uncertainty in the frame synchronization. Second, even if no error occurred in frame synchronization, there would still be significant errors at a data demodulator. What is needed in the art is a system to recover timing and data in an IBOC system subject to timing and frequency offsets.
Prior art related to OFDM includes the following:
U.S. Pat. No. 5,732,113 entitled “Timing and Frequency Synchronization of OFDM Signals” to T. M. Schmidl et al., issued Mar. 24, 1998 (Schmidl), discloses timing, carrier frequency, and sampling rate synchronization of a receiver to an OFDM signal using two OFDM training signals. A first OFDM training signal has only even numbered sub-carriers, and substantially no odd numbered sub-carriers, an arrangement that results in half-symbol symmetry. A second OFDM training symbol has even numbered sub-carriers differentially modulated relative to those of the first OFDM training signal by a predetermined sequence. Synchronization is achieved by computing metrics, which utilize the unique properties of these two OFDM training signals.
U.S. Pat. No. 4,604,583 entitled “Frequency Offset Correcting Circuit” to H. Aoyagi et al., issued Aug. 5, 1986 (Aoyagi), discloses a frequency offset correction circuit in a demodulator which receives parallel channel signals and recovers each baseband signal of the parallel channel. The frequency offset correction circuit includes a second order phase-locked loop following a demodulating circuit arranged to detect baseband signals of incoming parallel channel signals. The second order phase-locked loop include first and second control loops. The first control loop corrects a static phase shift of the pilot channel while the second control loop functions to correct abrupt frequency offset. The second order phase-locked loop is further utilized to correct both static phase shift and abrupt frequency offset data channels. A third control loop compensates for static or slowly changing frequency offsets of the channels.
U.S. Pat. No. 5,444,697 entitled “Method and Apparatus For Frame Synchronization in Mobile OFDM Data Communications” to C. Leung et al. (Leung), discloses a method and apparatus for achieving symbol (frame) synchronization of digital data in an OFDM channel. A three-stage synchronization process is disclosed. First, an onset of an OFDM frame is detected. Second, coarse synchronization is achieved by sampling the received signal, and measuring the correlation between the received signal and a reference signal. Third, synchronization is achieved by calculating the time shift between the coarse synchronization point and the actual synchronization point and using the calculated time shift to determine the phase correction to apply to each data-carrying sub-carrier. The transmitted data is recovered by decoding the information obtained about the phase and amplitude of the data-carrying sub-carriers.
U.S. Pat. No. 5,771,224 entitled “An Orthogonal Frequency Division Multiplexing Transmission System and Transmitter and Receiver Therefor” to T. Seki et al. (Seki), issued Jun. 23, 1998, discloses transmitting an OFDM frame with null symbols and reference signals placed at the beginning portion of the frame, and QPSK symbols placed in an information symbol data region in the frame with equal spacing in time and frequency. At a receiving end, an error detector detects amplitudes and phase errors of each carrier from the reference symbols and a variation detector detects the variations in the amplitude and phase of a received signal from the QPSK signals. The carrier amplitude and phase errors are corrected by a correction information producing section on the amplitude and phase variations of the received signals detected by the variation detector to produce correction information. An equalizing section equalizes the demodulated symbol data according to the correction information.
None of the prior art discloses correcting timing and frequency offset in OFDM systems working in the time domain after achieving frame synchronization by using an auto correlation function whereby nearly perfect cancellation of noise and multipath fading is achieved over a range of offset values.